1. Field of the Invention
The present invention relates to the field of the methods for the analysis and/or monitoring of the partial discharge behavior of an electrical operating means, in particular in terms of its development over time.
2. Discussion of Background
Partial discharges (TE) designate those electrical discharge phenomena which do not lead to the breakdown of the entire insulation section. They are propagated only in a spatially limited region.
Partial discharges always take place within a gas chamber or else at interfaces between a gas chamber and other surfaces. They can also occur in solid insulation as a consequence of highly inhomogeneous field conditions. However, they generally occur far above the technical limits of the use of the insulating materials, and are therefore generally not relevant to the normal operating case.
Partial discharges form as a consequence of inhomogeneous field conditions, for example on account of different dielectric constants of the insulating materials, and the possible local overloading of individual sections of the insulating material which result from this.
Such partial discharges always lead to damage to the installation surrounding them. On the other hand, the extent to which this damage leads to significant aging of the surrounding medium depends on a large number of parameters.
For example, the partial discharges can be of such low energy in terms of intensity that the degree of damage, measured over technically relevant time periods, can be disregarded. Likewise, the surrounding medium (insulating material) can be resistant to partial discharges in such a way that the damaging effect, within relevant time periods, can likewise be disregarded, even given an increased partial discharge intensity.
Likewise, the insulating material can be composed in such a way that damaged “portions” are led away from the location of the discharge activity, for example by means of natural convection. This is possible in the case of circulating air as insulating material or else in the case of circulating liquids, and is also used technically there.
The detection of such discharges can be carried out in various ways, such as by means of optical and electromagnetic detection processes or else by means of acoustic processes. In order to detect partial discharges within spatially extended arrangements, the electrical detection of these discharges by means of measuring the partial discharge current has become widespread. Enclosed insulating material portions can be monitored sufficiently reliably only by means of measurements of the electrical signals.
Measuring instruments for detecting such phenomena are widespread. They are capable of recording the partial discharge activity over a specific time period and displaying it. Likewise, such instruments are capable of carrying out measurements upon request or periodic measurements independently in the event that remote control ability is implemented. Measurements are also frequently carried out periodically, by a transportable partial discharge measuring instrument being connected at regular time intervals to the object to be measured.
The feature common to these methods is frequently that the measurements can generally be carried out only at relatively long time intervals, for example every six months, because of the large amounts of data which accumulate. In addition, such measurements are frequently carried out on site by specifically trained personnel, so that the repeated measurements cannot be carried out too frequently either for reasons of cost.
However, it is also important that very many partial discharge processes develop relatively slowly, so that repeated measurements often cannot be justified in every case.
The disadvantages with periodic measurements are:    1. Rapid changes in the partial discharge behavior are consequently not detected. Thus, this information, which may be important under certain circumstances, is not available.    2. Information relating to the partial discharge behavior is not available under all possible operating statements (load, temperature, vibration, . . . ). This is likewise equivalent to the loss of specific, to some extent important, information.    3. Measures based on changing partial discharge behavior are, under certain circumstances, introduced too late or else critical damage is detected too late.
A remedy is provided by systems which observe the partial discharge behavior continuously and    1. store the data in compressed form or detect and store it only over short intervals, or systems    2. which subject the currently measured partial discharge behavior to a superimposed process and, from this process, extract a few characteristic values which represent the partial discharge behavior adequately, in order then to store these characteristic values. These characteristic values can subsequently be used to initiate further actions.
Methods which subject the measured partial discharge behavior to a superimposed process are outlined in U.S. Pat. Nos. 6,088,658 and 6,192,317. These patents present a method which, in principle, operates in accordance with the following scheme:    1. A system is trained with a number of items of partial discharge data in order to form classes for known fault types.    2. The state of the electrical operating means, from which this partial discharge data has been collected, must be known.    3. In order to train the system, at least two data sets are needed: the partial discharge data of insulation damaged in any way, and also the partial discharge data relating to an operating means with sound insulation. With this data, at least two classes are formed; the state of damage must be known in each case.    4. Forming classes means:            a) An insulation state is determined which is representative of the class and which represents this insulation state (class) on average, and        b) Threshold values are defined, within which this class is representative.            5. Further partial discharge data is then supplied to the system in order to form further classes of known insulation states and in order once more to obtain threshold values relating to these. However, the system is in principle able to manage with only two classes.    6. In order to determine the state of an insulation with the aid of the partial discharge measurement, the spacing (distance) of the current partial discharge measurement is obtained with the representative partial discharge measurement (representative measurement) of each previously formed class.    7. Following the comparison of these calculated distances with the associated threshold values, the current measurement is allocated to the class whose spacing from the representative measurement is the lowest, if the spacing from the class lies within the threshold value. Otherwise, the current measurement counts as invalid or not to be allocated.
Methods which use these or similar approaches are identified by the following characteristics:    1. The classes must have been formed with a number of items of known partial discharge data before reliable output values can be expected. This number of items of representative data is important in particular in order to define the thresholds within which the classes are detected. This is also important if the partial discharge data is superimposed, for example by interfering data and noise, which in each case make the formation of classes more difficult.    2. Only insulation states which have previously been trained for the class formation are detected.    3. If, during the training, the installation state was not taken into account correctly, then the output value from this process will always be affected by errors or allocated to the classes formed erroneously.    4. A comparison of the partial discharge behavior of different electrical operating means is not always possible if the peripherals of the operating means or the operating means themselves are not identical. The data base which is used to form the classes may therefore be thin or less reliable.    5. However, if the classes formed represent machines of different design and different peripherals, then access is automatically made to a very large data base, which improves the quality of the statements.    6. If sufficiently many fault types have been used to form various classes, then the fault types can also be detected in a differentiated manner.    7. The results obtained from the class formation and from the comparison with newly obtained partial discharge data can be used to document the partial discharge history of an electrical operating means and to make a future prognosis.